Spruce, Dwarf Alberta
Plant of the Week
Dwarf Alberta Spruce
Latin: Picea glauca 'Conica'
Winter has finally arrived. Two days ago, I watched a barefoot boy play football with his pals on a balmy January morning; within a few hours, the temperature had plummeted almost 70 degrees with the mercury hovering near zero. Such swings can't be good people or their plants.
Plants have adapted mechanisms to cope with the cold of winter so I thought it might be interesting to consider how some of the most cold hardy plants adapt to their chilly haunts. One of the most cold-tolerant plants we commonly see in our landscapes is the Dwarf Alberta Spruce (Picea glauca 'Conica').
This dwarf tree can reach 15 feet tall in a perfect location in the northland where conifers grow best, but most of us are more familiar with it around the holiday season when mass merchants sell thousands of the foot tall, perfectly conical living Christmas trees as tabletop decorations. The trees maintain that perfect conical form as they slowly increase in size over the years.
The needles are light green, about a half-inch long and radiate at all angles around the stem, giving a bottlebrush effect to the small branches. It's a genetic mutant that doesn't produce cones as would be expected with most spruce.
Dwarf Alberta Spruce was discovered in 1904 in the Northern Rockies near Lake Laggan, Alberta as two Arnold Arboretum botanists awaited the arrival of their train to take them back to Boston. Today, they would meet with serious government red tape and social castigation, but at the time they spotted the stunted seedling and recognized it as something new and different. Without a second thought, they dug it up and took it home with them.
Back in Boston, the dwarf spruce was found to be easy to propagate and was released to the nursery trade a few years later.
The most famous of the pair was Alfred Rehder, who became known to thousands of students for his Manual of Cultivated Trees and Shrubs. In the1940 edition, Rehder was the first to use a plant hardiness zone map based on minimum winter temperatures. His first hardiness map had only seven zones but served as the precursor to the 12-zone USDA map we use today.
Using the modern version of the hardiness zone map, the Dwarf Alberta Spruce will grow from zone 2 to 7. The zone 2 rating means that this diminutive tree will tolerate temperatures as low as -40 degrees Fahrenheit. Ironically, at this cold temperature the centigrade and Fahrenheit temperature scales cross, and quite by accident, it represents an important biological juncture as well.
Minus 40 degrees is the lowest temperature that pure water can remain in a liquid state. Common experience tells us that water freezes at 32 degrees Fahrenheit, but it was the German inventor of the mercury thermometer, Daniel Fahrenheit (1686-1736) who first demonstrated that pure water could go to colder temperatures before freezing.
To freeze, water needs what is called an "ice nucleation agent" - something to begin the crystallization process. Plants that tolerate very cold temperatures use membranes to separate the liquid water in their cells from these nucleation agents, thus they can grow in very cold environments.
The timberline at the top of a mountain, usually between 13,000 and 14,000 feet, is a natural marker for this -40 degree temperature zone. All hardy woody plants, to a lesser or greater degree, have the ability to create these reservoirs of super pure water in their cells, but to do so takes time.
Rapid temperature drops can freeze the water in the cells before it has a chance to undergo purification. The ice crystals puncture membranes and the cells die.
The Dwarf Alberta Spruce will grow in Arkansas and is occasionally seen as far south as Little Rock.
Summer heat and humidity are not to its liking, so try to locate it where it gets some afternoon shade but otherwise good light. The north or east side of the house is probably the best location. Make sure it's planted in a fertile, well drained soil not allowed to get too dry during the summer. In our heat, spider mites can be a serious problem, so inspect the plants in June and July and spray if needed.
By: Gerald Klingaman, retired
Extension Horticulturist - Ornamentals
Extension News - January 9, 2004
The University of Arkansas Division of Agriculture does not maintain lists of retail outlets where these plants can be purchased. Please check your local nursery or other retail outlets to ask about the availability of these plants for your growing area.